1. Field of the Invention
The present invention relates to a tubular evanescent wave sensor for molecular absorption spectroscopy. It more particularly applies to the analysis of species in a liquid solution.
2. Discussion of Background
Molecular absorption spectroscopy is an analytical method very frequently used on the laboratory scale. It is based on the selective absorption of light radiation by the species to be analyzed in a solution.
The principles of this method will now be defined. I.sub.O,.lambda. is the intensity of the incident radiation and I.sub.t,.lambda. the intensity of the radiation transmitted at wavelength .lambda.. These two parameters are linked for the formula: EQU I.sub.T,.lambda. =I.sub.O,.lambda. 0.10.sup.-DO
in which DO represents the optical density equal to .epsilon..sub..lambda. LC, in which: .epsilon..sub..lambda. is the molar extinction coefficient (expressed in mol.sup.-1. liter. cm.sup.-1), L is the optical path in the solution (expressed in cm), and C is the concentration of the chemical species to be analyzed (expressed in mol.liter.sup.-1).
For spectrophotometers commonly used in the laboratory, the highest measurable optical densities are between 4 and 7 (giving an attenuation of the signal varying between 10.sup.4 and 10.sup.7).
Using measuring cells with a reduced optical path L, it is still possible to measure absorbances .epsilon..sub..lambda. C which can reach 100 cm.sup.-1. Beyond these values, the sample to be analyzed must be diluted, provided that the dilution medium does not disturb the optical properties of the species to be analyzed.
In line analysis by molecular absorption spectroscopy (the term molecular absorption photometry also being used) requires the use of an optical sensor connected to the measuring apparatus by means of two optical fibers. One of the fibers makes it possible to carry the light from the light source of the apparatus up to the measurement point. The other optical fiber makes it possible to collect the transmitted radiation after an optical path of length L in the solution.
As in the case of a laboratory analysis, the optical path is adjusted as a function of the opacity of the solution to be analyzed.
However, when the absorbance is very high (well above 50 cm.sup.-1), the optical path necessary for the measurement becomes so small that the solution is trapped between the light emitter and the light collector associated with the sensor due to a capillarity phenomenon. Thus, the sensor no longer serves as an inline sensor.
Hereinafter consideration is given to evanescent wave sensors, which are used in absorption measurements of highly absorbant solutions. Their principle is based on the submicron intrusion property of light at the instant of its deviation by a reflecting surface.
The notion of the evanescent wave will be defined. When a light ray coming from a first medium with a refractive index n.sub.1 arrives at the surface of a second medium of refractive index n.sub.2 lower than n.sub.1, the incidence angle .beta. of the light ray determines two behaviours of the corresponding electromagnetic wave:
If .beta. is lower than a critical angle .beta..sub.c, the electromagnetic wave is completely transmitted to the second medium.
If .beta. is equal to or greater than .beta..sub.c, the electromagnetic wave is completely reflected at the interface between the two media.
The critical angle .beta..sub.c is defined by the following formula: EQU sin .beta..sub.c =n.sub.2 /n.sub.1.
From the physical standpoint, the sudden passage from a transmitted wave to a reflected wave is not completely satisfactory.
A more precise study using Maxwell equations introduces a transient stage between these two phenomena using the notion of the evanescent wave.
The modelling of the behaviour of electromagnetic waves at the medium change shows that in the case where .beta. is equal to or greater than .beta..sub.c, the light slightly penetrates the second medium before being reflected towards the first. This intrusion, whose depth is linked with the wavelength .lambda. of the radiation, is known as the "evanescent wave".
The intrusion depth dp is given by the following formula: ##EQU1##
For wavelengths in the visible range (0.4 to 0.8 .mu.m), said depth dp is a few micrometers. It is this property which is used in evanescent wave sensors.
The interest of an evanescent wave in analysis will be demonstrated hereinafter. An electromagnetic wave propagates in an optical conductor effecting a plurality of reflections. For each reflection, the surrounding medium is probed by the radiation. It is therefore possible to take advantage of these reflections to produce an optical sensor.
However, the dimensions of the sensor would be highly dependent on the incidence angles of the wave.
A curve illustrating the penetration of a wave as a function of its incidence angle is given in FIG. 1. The incidence angle .beta. is plotted on the abscissa and the intrusion depth dp on the ordinate.
Two areas can be observed in FIG. 1 being separated by a value .beta.1 of angle .beta., which is dependent on the wavelength of the radiation used and the absorbance of the medium probed by the radiation:
an area I, characterized by a high wave penetration, which generates a high reflection level per unit of length of the optical conductor and PA1 an area II, characterized by a low wave penetration, which generates a low reflection level per unit of length of the optical conductor. PA1 of the distance separating the light inlet from the outlet, PA1 their overall dimensions (for the sensors illustrated in FIG. 2), PA1 and the difficulties in implementing and replacing (for the sensors illustrated in FIG. 3). PA1 emission-reception means for emitting a light, which is able to interact with the fluid, and receives said light after its interaction with said fluid, PA1 a tubular light guide intended to be immersed in the fluid and whereof a first end is positioned facing the emission-reception means and whereof a second end is able to reflect the light propagating in the guide and PA1 means for maintaining the guide at a distance from the emission-reception means, said distance permitting the formation of the evanescent wave in the fluid when the light propagates in the guide. PA1 a central optical fiber intended to emit the light, the end of said fibre positioned facing the tubular light guide and the latter being coaxial and PA1 peripheral optical fibers surrounding the central optical-fibers and intended to receive the light passing out of the first end of the guide.
It is therefore area I which offers the most advantages (small overall dimensions) for implementing a miniature evanescent wave sensor according to the invention, said sensor being intended to function in area I.
However, it is necessary to be able to accurately adjust the propagation angles of the electromagnetic wave.
As soon as it is possible to adjust the propagation angles of an electromagnetic wave in an optical guide, it is possible to control the depths probed at each of the reflections and consequently have the optical guide serving as an optical evanescent wave sensor.
Such depths of approximately 1 .mu.m for wavelengths in the visible range open new perspectives to in situ spectrometric analytical methods.
At present numerous laboratories are showing great interest in such sensors. However, the problems of designing such sensors have only been studied to a limited extent.
Two categories of optical evanescent wave sensors or probes are known.
In the first category, diagrammatically illustrated in FIG. 2, the sensors comprise an optical fiber 2 without a protective sheath and placed in a branch circuit 4 in which circulates the solution to be analyzed in the direction indicated by the arrows F. FIG. 2 also shows a light source 6 and a light detector 8 respectively coupled to the ends of the fiber 2.
In the second category diagrammatically illustrated in FIG. 3, the sensors comprise a glass plate 10, whereof one face is immersed in the solution to be analyzed 12.
A longitudinal slot made on the duct 13 in which circulates the solution in accordance with arrows G receives the glass plate 10 in order to permit said immersion. The opposite plate face is provided with two prisms 14 and 16 at its two ends. These are respectively intended to inject the electromagnetic wave into the plate 10 and extract said wave from the plate.
Moreover, these prisms 14 and 16 are respectively coupled to a light source 18 and to a light detector 20.
The sensors shown in FIGS. 2 and 3 are the most widely used on the laboratory scale. However, they are not very suitable for the industrial environment because